That may have been to some extent true when he wrote the Book of Lost Tales, but the Silmarillion and LotR are quite divorced from that and it has never been intended to glorify paganism.I thought that his main idea was to create a replacement mythology for the British Isles because the Normans destroyed the records.
That would make it sort of pagan fanfiction.
That isn't a reason for Eru to allow the gate to form, it's a reason for him to keep their discordant influence out of his choir. Melkor's themes are already filling any purpose that could serve.Aside from the Apostles, the Gate Gods have no dominion outside their world and can't go to Arda. But just think about it. Rory traveling with Gandalf who already has a bit of a dodgy reputation. Just imagine it.